ABSTRACT
The major general-purpose Monte Carlo particle transport codes (i.e., EGS4, FLUKA, Geant4, MCNP) have been capable of transporting charged particles in the presence of external electromagnetic fields for many years. At the time of publication of the first version of this book (2013), there had been little reason in radiotherapy to specifically include magnetic field effects in Monte Carlo modelling. This book itself only contained several short sections that described the use of magnetic fields, for example, modelling scanning magnets in a proton beam line (Peterson 2009a, b), effects on electron beam transport near a linac head bending magnet (O’Shea, 2011), and finally a mention of GPUMCD as a code being able to model patient doses in the presence of uniform magnetic fields, such as those from a prototype MRI-Linac system (Hissoiny et al., 2011). At that point in time, the reader may not have been aware of the true extent, however, of what was occurring in the field of MRI-guided radiotherapy, and so the content of this book reflected this. Fast forward 7 years to the year 2020, and we are witnessing the evolution of a major player in the field of advanced image-guided radiotherapy with patients being treated with integrated MRI-Linacs systems (Mutic and Dempsey, 2014; Lagendijk et al., 2014). It is without question that Monte Carlo methods have been a solid pillar in many aspects of supporting and maintaining the growth of real-time MRI-guided radiotherapy with MRI-Linac systems. These include predicting and/or confirming complicated patient dose changes, dosimetry, and radiation beam production issues, all due to the presence of the magnetic field from MRI scanners. This chapter will provide an overview of the basics of charged particle transport specifically inside magnetic fields and describe various applications, with particular attention to MRI-guided radiotherapy. An overview of the interesting history of Monte Carlo-based studies of therapeutic X-ray and electron beams subject to magnetic fields is also presented.
